The technique for controlling the directivity of audio signal propagation by using a plurality of loudspeakers arranged in an array form has conventionally been proposed (e.g., patent document 1).
FIG. 10 is a diagram explaining a basic principle of this technique. This diagram shows an example in which audio signals are outputted from a plurality of small loudspeakers arranged in a linear form, and control is provided such that the audio signals are directed toward a focal point F. Although the same audio signals are outputted from the respective loudspeakers, at that time a delay is provided to allow the audio signals from the respective loudspeakers to reach the focal point F at the same time. By providing such control, it is possible to form a sound beam having directivity only in a fixed direction with a sound pressure distribution such as the one shown in FIG. 11. By setting the focal point F in a wall surface direction, it is possible to form a virtual sound source in the wall surface direction with respect to a listener who receives the sound beam reflected from the wall surface.
To provide the above-described delay time control, an audio signal processing unit such as the one shown in FIG. 12 is connected to the loud speaker array of FIG. 10. The audio signal is inputted to and delayed by a delay circuit, and audio signals are fetched from taps T(N), T(N+1), . . . , with a predetermined amount of delay corresponding to each loudspeaker. The fetched audio signals are multiplied by gain coefficients by coefficient multipliers 101(N), 101(N+1), . . . , are amplified by amplifiers 102(N), 102(N+1), . . . , and are then outputted as sound. The gain coefficient which is multiplied by the coefficient multiplier is a window function or the like.
If the loudspeakers are arranged horizontally in a line form, as shown in FIG. 10, directivity control in an arbitrary direction in the horizontal direction is possible, and the directivity becomes wide (cone beam) in the vertical direction. In addition, if the loudspeakers are arranged in the form of a horizontal and vertical matrix, directivity control in an arbitrary direction is possible both in the horizontal direction and in the vertical direction.
The directivity of sound is controlled by using the array loudspeaker in the above-described manner, and a virtual sound source can be set in the direction of the wall surface distant from the loudspeaker. Further, a multi-channel virtual sound source can be formed by one (one set of) array loudspeaker by separately forming a plurality of beams. Therefore, this system is suitable for a case in which a multi-channel source such as the 5.1 channel system whose practical use is underway is realized by a simple configuration of an audio system.
Patent document 1: WO 01/23104 A2
However, this system has the following problems.
The minimum frequency for which directivity control can be provided by the array loudspeaker is determined by the entire width of the array. Namely, a width which is several times the wavelength is required to provide satisfactory control, so that since the wavelength is 30 cm in the case of 1 kHz, for example, it is desirable to secure a width of 1 m or thereabouts.
On the other hand, the maximum frequency which can be controlled is determined by the interval (pitch) between the small loudspeakers (loudspeaker elements). If the wavelength becomes shorter than the pitch, a grating lobe is formed, i.e., a beam is formed in a direction other than intended.
Accordingly, the diameter of the loudspeaker elements and the pitch between the elements should desirably be as small as possible. However, if the loudspeaker is made compact to make the pitch short, the inputtable power is small, and the conversion efficiency is poor, so that there has been a problem in that the output sound volume becomes insufficient.
In addition, while the loudspeaker is made compact and the pitch is made small to control a wide frequency band, if the array width is made large, the number of loudspeakers must be increased, so that there has been a problem in that the apparatus becomes large in size. If an attempt is made to provide three-dimensional control by arranging the loudspeakers in a planar form, there has been a problem in that the apparatus becomes larger in scale.
Meanwhile, if consideration is given to practical advantages, directivity control in the horizontal direction is very useful, but the advantages derived from directivity control in the vertical direction are relatively small. A human being has a high sound source recognition sensitivity in the horizontal direction through binaural processing, and the horizontal plane processing is also a basis of the surround sound source such as the 5.1 channel system. On the other hand, if a beam having a narrow directivity in the vertical direction is formed, the direction of the beam must be changed depending on whether the user is sitting, standing, or sleeping. Furthermore, if a plurality of users are listening with different postures, it has impossible to allow all the users to listen with the same sound quality. Moreover, consideration is given to the introduction into the users' rooms having different shapes, it is difficult to optimally adjust a three-dimensional beam path, so that horizontal plane control, for which only the angle in the focal direction needs to be adjusted, is practical in use.
Accordingly, it is conceivable to provide beam control only in the horizontal direction by the line array; however, if the line array is adopted, the number of loudspeaker elements decreases, so that the input power still remains as a problem.
The input power of a popular full-range loudspeaker of 3 cm or less is 2 W or thereabouts, and if 20 such loudspeakers are arranged as a line array, the input power totals only 40 W. Although this power presents no problem as an ordinary television loudspeaker, it is insufficient as the total power for a multi-channel audio loudspeaker. In addition, in the case where a beam is formed, a window function or the like is multiplied, there can be no cases where all the loudspeakers are operated with full power.